A conventional turbomachine essentially comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing connected downstream of an engine outlet manifold. In a turbocharger, rotation of the turbine wheel drives a compressor wheel mounted on the other end of the shaft within a compressor housing to deliver compressed air to the engine intake manifold. In a power turbine, rotation of the turbine wheel drives a gear which transmits mechanical power to the engine flywheel or crankshaft. The turbomachine shaft is conventionally supported by a bearing assembly made up of journal and thrust bearings, including appropriate lubricating systems, located within a bearing housing.
The journal bearings support the turbomachine shaft for rotation and typically consist of a pair of fully floating bearings, each in the form of an annular bush mounted on to the shaft. One journal bearing is usually located at each end of the section of the shaft received in the bearing housing and is retained in place axially relative to the shaft by a pair of circlips or the like, one circlip provided to each side of the bearing.
Axial forces imparted to the turbomachine shaft during operation are resisted by an axial thrust bearing that is typically in the form of a thin disc disposed around the shaft and supported on one side by a thrust collar and on the other by the bearing housing and/or other components. The thrust bearing has a central bore for receiving the thrust collar that is mounted on the shaft for rotation therewith immediately adjacent a radial step defined thereon. The thrust bearing is supported between a retaining circlip and the bearing housing. Axial forces towards the turbine wheel are transmitted to the thrust bearing by the thrust collar and in the opposing direction by either an oil slinger mounted on the turbomachine shaft in a turbocharger or a thrust ring mounted on the turbomachine shaft in a power turbine.
The turbomachine shaft and bearing assembly rotate at very high speeds and effective lubrication is imperative to avoid premature failure through wear or seizure. Lubricating oil is supplied to the bearing assembly from the engine oil system via an oil inlet in the bearing housing. Oil is distributed via galleries and passages in the bearing housing to circumferential holes in the outer races of journal bearings. When the engine is in operation the oil is supplied under pressure to the rotating bearing assemblies and the oil penetrates through the circumferential holes to an interface between the inner part of the bearings and the shaft. Similarly, the oil is supplied from the galleries and passages to the periphery of the thrust bearing from where it penetrates through a radially extending passage in the disc to the interface between it and the thrust collar. The oil drains from the bearing assembly bore between the thrust bearing and thrust collar and either the oil slinger in a turbocharger or the thrust ring in a power turbine, and at the end of the bearing housing bore adjacent to the turbine housing.
Typically, turbomachine journal bearings have been manufactured from a leaded bronze in view of the excellent suitability of this class of alloys to the harsh working environment found in turbomachines. An exemplary leaded bronze suitable for use in a turbocharger journal bearing has the following chemical composition: Sn 5-7 wt %; Pb 14-18 wt %; Ni 0.75 wt % max; Fe 0.4 wt % max; P 0.05 wt % max; Zn 1.5 wt % max; and Cu balance. More recently, increasingly stringent industry regulations have resulted in the need to find alternative materials that contain less or, preferably, no lead in view of its toxicity and the potential harm that it can cause to the environment if released during manufacture, use or subsequent disposal of components incorporating it. This presents numerous challenges, however, in view of the excellent chemical and physical properties that result from the inclusion of lead in turbomachine journal bearings.